Mars Global Surveyor
Mars Orbiter Camera

Dark Dunes Over-riding Bright Dunes

MGS MOC Release No. MOC2-201, 31 January 2000

Some martian sand dunes may be more active than others. In this
picture, wind has caused the dark and somewhat crescent-shaped dunes
to advance toward the lower left. While their movement cannot actually
be seen in this April 1998 snapshot, the location of their steepest
slopes--their slip faces--on their southwestern sides
indicates the direction of movement. Oddly, these dark dunes have
moved across and partly cover sets of smaller, bright ridges that also
formed by wind action.

This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image
illustrates an intriguing martian "find". Strangely, the two dune
types have different shapes and a different relative brightness.
There are two explanations for the relationship seen here, and neither
can be distinguished as "the answer"---(1) it is possible that the
brighter dunes are old and cemented, and represent some ancient wind
activity, whereas the dark dunes are modern and are marching across
the older, "fossilized" dune forms, or (2) the bright dunes are
composed of grains that are much larger or more dense than those that
compose the dark dunes. In the latter scenario, the bright dunes move
more slowly and are over-taken by the dark dunes because their grains
are harder to transport. An interpretation involving larger or denser
grains is consistent with the small size and even-spacing of the
bright dunes, as well, but usually on Earth such features occur
on the surfaces of larger, finer-grained dunes, not
under them. The actual composition of either the bright or
dark materials are unknown. This example is located on the floor of an
impact crater in western Arabia Terra at 10.7°N, 351.0°W.
The picture is illuminated from the right.

Image credit: NASA/JPL/Malin Space Science Systems

Malin Space Science Systems and the California Institute of Technology
built the MOC using spare hardware from the Mars Observer mission. MSSS
operates the camera from its facilities in San Diego, CA. The Jet Propulsion
Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor
spacecraft with its industrial partner, Lockheed Martin Astronautics, from
facilities in Pasadena, CA and Denver, CO.